Apple:

The phrase “Hey Siri” was originally chosen to be as natural as possible; in fact, it was so natural that even before this feature was introduced, users would invoke Siri using the home button and inadvertently prepend their requests with the words, “Hey Siri.” Its brevity and ease of articulation, however, bring to bear additional challenges. In particular, our early offline experiments showed, for a reasonable rate of correctly accepted invocations, an unacceptable number of unintended activations.

Another fascinating article in Apple’s Machine Learning series. It got a bit dense for me at times (or, rather, I’m a bit dense for the article) but there are lots of interesting tidbits of info included.